Re: SIM only - is the monthly charge (not usage) paid in advance?
You're indeed correct.
You'd be billed within the first week of your new SIM deal and the payment would go out around 14 days later, this will be your set bill and payment dates for your account unless you request to change your billing dates.
Any usage would indeed be added onto your next month's bill. Your final bill will only be over-allowance costs because with mobile it takes into account cancellation requests on your next bill.